Current standards of voice transmission refer to wideband (WB) audio signals. For example the standard G722 refers to a 16 KHz sampling rate (8 KHz spectrum bandwidth) with division to two sub bands (SB) of 4 KHz each.
The communication according to the standards, such as G722 includes transformation of the analog signal from the speakers end into digital signal, splitting the WB into two SB, encoding each of the two SB and transmission of the encoded sub bands over the network.
The traditional approach at the receiving end suggests decoding both encoded sub bands and merging the two sub bands. However this approach further requires processing a signal that is sampled at a relatively high sampling frequency. For example if this approach is applied on a G722 system, then the merged signal should be sampled at the rate of 16 KHz.
To reduce the processing load, the signal may be decoded on each of the two sub bands, but not merged immediately thereafter and by that enabling to perform echo cancelling and various DSP (digital signal processing) on low frequency sub-bands. However if the sub bands are not merged immediately after decoding, each of the sub-band signals is carrying a significant non linear aliasing component. Thus, the advantage of low sampling frequency causes a non linear aliasing problem.
The processing of two sub bands in half sampling rate provides a benefit only if there is an efficient and low cost method to suppress the non linear aliasing component that is contained in each of the two sub bands.